The present invention relates to read-only memories.
The prior art of read-only memories has used a very wide variety of techniques to encode information. However, whatever technique is used, both the row density and the column density provide limitations on the density of information of which can be stored.
Thus it is an object of the present invention to provide a read only memory with enhanced row density.
In conventional memories, the row density cannot be increased beyond a certain point, simply due to the limitations of available interconnect technology. That is, the permissible pitch of metal lines is limited not only by processing difficulties, which may be expected to ease gradually, but it is also limited by the current densities which can be used in a given material. Similar limitations apply to any material used for row lines in the prior art, whether this be a diffusion in the substrate, or polysilicon, or an additional metal level.
Thus it is an object of the present invention to provide a read-only memory which has row densities in excess of those permitted by prior art interconnect technology.
It is a further object of the present invention to provide a read-only memory having a bit density in excess of that permitted by future versions of conventional technology.
To achieve these and other objects, the present invention teaches a read-only memory wherein the rows comprise chains of quantum wells. Each quantum well is a potential well, e.g. an island of GaAs in an AlGaAs lattice, which is small enough that the energy levels of carriers within the wells are discretely quantized. The wells are very small, having a minimum dimension far less than a 1/10 of a micron, and the distance between adjacent wells in a chain is comparable to the maximum dimension of each of the wells. This means that, when the bias between two adjacent wells is adjusted to align the permissible energy levels of carriers in each of the two wells, tunneling will occur very rapidly (resonant tunneling) whereas when energy levels are not aligned, tunneling will be greatly reduced.
In the present invention, column lines are selectively coupled to the row lines they pass over by varying the degree of electrical coupling. Thus, a column line which is electrically coupled to some point in a chain of quantum wells will perturb the alignment of energy levels, and thus tremendously reduce the current through the chain of potential wells, when the voltage on the column changes from some preset level. However, if the column is not electrically coupled to the row line, the electrical alignment of adjacent wells will not be disturbed and resonant tunneling will continue regardless of a signal placed on the decoupled column line. Thus, electrical coupling is programmed in by such means as small holes cut in a metal field plate. When the addressed column line is changed from some reference voltage level, the current in each row line indicates whether that row line was coupled to the column addressed.
According to the present invention, to achieve the above and other objects, there is provided:
A read-only memory comprising:
a plurality of chains of quantum wells, each said chain comprising an alternate series of first and second potential wells,
said first and second potential wells each comprising an island of a semiconducting material having a minimum dimension less than 500 Angstroms and another dimension less than 1000 Angstroms;
a barrier medium, interposed between each adjacent pair of said first and second wells, wherein the minimum potential energy of carriers is at least 50 millielectronvolts higher than the minimum potential energy of carriers within said wells, said wells being physically separated by a distance which is less than three times the smallest physical dimension of either of said wells;
means for inputting carriers into each said chain of wells;
means for removing carriers from each said chain of wells; and
a plurality of conductors orthogonal to said plurality of chains of wells, wherein each said conductor is selectively either strongly or weakly coupled to plural ones of said chains of wells, in accordance with predetermined information, to encode said predetermined information.